1. Field of the Invention
The present invention is related to a liquid crystal display(LCD), and more particularly to a LCD having an improved viewing angle and transmittance.
2. Related Art
A liquid crystal display(LCD) has an optical aniostropy owing to a structure thereof. The optical anisotropy(or refraction index anisotropy) means that a refraction index at a major axis of a liquid crystal molecule is different from that at a minor axis of the liquid crystal molecule. Because of the optical aniostropy, a polarization of the light passing through the liquid crystal molecule and oscillating direction of the polarized light are varied. The refraction index is represented as a difference between a refraction index in a direction parallel to the major axis of the liquid crystal molecule(ordinary refraction index) and a refraction index in another direction perpendicular to the major axis thereof(extraordinary refraction index), which is called as a birefringence. On the other hand, in applying a voltage to the liquid crystal cell, the liquid crystal molecules are rearranged owing to a dielectric anisotropy of the liquid crystal and light passes through the rearranged liquid crystal molecules.
FIGS. 1A and 1B show an electrically controlled birefringence(ECB) mode liquid crystal display(LCD) using the birefringence of the liquid crystal molecule, wherein the LCD in FIG. 1A is under no voltage applied thereto and the LCD in FIG. 1B is subjected to the applied voltage.
The liquid crystal molecules 8 are formed between a upper substrate 1 and a lower substrate 4. On an opposite surface of the upper substrate 1 and an opposite surface of the lower substrate 1, are respectively formed a counter electrode 2 and a pixel electrode 5 on which vertical alignment layers 3 and 6 are disposed. On a non-opposite surface of the upper substrate 1 and a non-opposite surface of the lower substrate 4, are disposed the respective polarization layers 7a and 7b. The vertical alignment layers 3 and 6 are rubbed so as to have a pretilt angle of about 85 to 89 degrees.
In no applied voltage, as shown in FIG. 1A, all the liquid crystal molecules of the ECB mode LCD are vertically arranged to the upper substrate and the lower substrate. In applying voltage to the LCD, as shown in FIG. 1B, the liquid crystal molecules which are directly contacted with the vertical alignment layers of the upper substrate and the lower substrate maintain their original state and the liquid crystal molecules between the most outer liquid crystal molecules are slantly arranged in a given direction.
The ECB mode LCD has a merit of high contrast ratio. However, since when applying voltage, the liquid crystal molecules in the middle part of the liquid crystal layer are oriented in one direction, a phase retardation is different according to the viewing angle. Accordingly, the transmittance is varied responsive to the viewing angle and thus the viewing angle becomes narrow. On the other hand, when no voltage is applied to the LCD, a complete dark state can be realized at a just front of the liquid crystal cell. However, at a side of the liquid crystal cell there is the phase retardation with respect to the front, so a leakage current of the LCD is generated.